#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
#include<algorithm>
using namespace std;
const int N = 210;
int n, s, c;
int a[N], space[N];
int main()
{
    cin >> n >> s >> c;
    for (int i = 0; i < c; i++) cin >> a[i];
    sort(a, a + c);
    int all = a[c - 1] - a[0] + 1;
    for (int i = 1; i < c; i++) space[i - 1] = a[i] - a[i - 1] - 1;
    sort(space, space + c - 1, greater<int>());
    for (int i = 0; i < n - 1; i++) all -= space[i];
    cout << all << endl;
}